The art of fabricating custom-fit prosthetics in the medical and dental fields is well-known. Prosthetics are replacements for tooth or bone structure; examples include restoratives, replacements, inlays, onlays, veneers, full and partial crowns, bridges, implants, posts, etc. Currently, most prostheses in dentistry are either made by hand by a dental practitioner while the patient is in the dental chair, or by an independent laboratory who is capable of such fabrication.
Materials used to make the prostheses typically include gold, ceramics, amalgam, porcelain and composites. For dental restorative work such as fillings, amalgam is a popular choice for its long life and low cost. Amalgam also provides a dental practitioner the capability of fitting and fabricating a dental filling during a single session with a patient. The aesthetic value of amalgam, however, is quite low, as its color drastically contrasts to that of natural teeth. For large inlays and fillings, gold is often used. However, similar to amalgam, gold fillings contrast to natural teeth hues. Thus, dental practitioners are increasingly turning to ceramic or polymer-ceramic composite materials whose color can be matched with that of the tooth.
The conventional procedure for producing dental prosthetics typically requires the patient to have at least two sessions with the dentist. First, an impression is taken of the dentition using an elastomeric material from which a cast model is made to replicate the dentition. The prosthetic is then produced from the model using metal, ceramic or a composite material. A series of steps for proper fit and comfort then follows. Thus, fabrication of custom prostheses involves intensive labor, a high degree of skill and craftsmanship, and lengthy times (1-2 days). Alternatively, a practitioner may opt for a sintered metal system that may be faster. However, those procedures are still labor intensive and complicated.
In recent years, technological advances have provided computer automated machinery capable of fabricating prostheses using minimal human labor and drastically lower work time. This is frequently referred to as “digital dentistry,” where computer automation is combined with optics, digitizing equipment, CAD/CAM (computer-aided design/computer aided machining) and mechanical milling tools. Examples of such a computer-aided milling machine include the CEREC 2™ machine supplied by Siemens (available from Sirona Dental Systems; Bensheim, Germany) VITA CELAY™, (available from Vita Zahn Fabrik; Bad Sackingen, Germany) PRO-CAM™ (Intra-Tech Dental Products, Dallas, Tex.) and PROCERA ALLCERAM™ (available from Nobel Biocare USA, Inc.; Westmont, Ill.). U.S. Pat. Nos. 4,837,732, 4,575,805 and 4,776,704 also disclose the technology of computer-aided milling machines for making dental prostheses. These machines produce dental prostheses by cutting, milling, and grinding the near-exact shape and morphology of a required restorative with greater speed and lower labor requirements than conventional hand-made procedures.
Fabrication of a prostheses using a CAD/CAM device requires a “mill blank,” a solid block of material from which the prosthetic is cut or carved. The mill blank is typically made of ceramic material. U.S. Pat. No. 4,615,678 discloses a blank adapted for use in machine fabrication of dental restorations comprising a ceramic silica material. There exist various mill blanks available commercially, including VITA CELAY™ porcelain blanks Vita Mark II Vitablocks™ and VITA IN-CERAM™ ceramic blanks (both available from Vita Zahn Fabrik; Bad Sackingen, Germany). Machinable micaceous ceramic blanks (e.g. Corning MACOR™ blanks and Dentsply DICOR™) are also known in the art.